1 Corinthians 15:25 - Albert Barnes' Notes on the Bible

For he must reign - It is fit, or proper (δει dei), that he should reign until this is accomplished. It is proper that the mediatorial kingdom should continue till this great work is effected. The word “must” here refers to the propriety of this continuance of his reign, and to the fact that this was contemplated and predicted as the work which he would accomplish. He came to subdue all his enemies; see –Psalms 2:6-10; or Psalms 110:1, “The Lord said unto my Lord, Sit thou at my right hand until I make thine enemies thy footstool.” Paul, doubtless, had this passage in his eye as affirming the necessity that he should reign until all his foes should be subdued. That this refers to the Messiah is abundantly clear from Matthew 22:44-45.